Electric furnace



1. FITZPATRICK AND L. T. STEPHENS.

ELECTRIC FURNACE.

APPLICATION msn MAR. 2|. |9194 1 ,3 1 3, 985 Patented Aug. 26, 1919.

UNITED STATES PATENT OFFICE.

JOHN FITZPATRICK AND LEVI THURSTON STEPHENS, OF NIAGARA FALLS, NEW YORK, ASSIGNORS T0 THE CARBORUNDUM COMPANY, 0F NIAGARA FALLS, NEW YORK, A

CORPORATION 0F PENNSYLVANIA.

ELECTRIC FURNACE.

Specification of Letters Patent.

Patented-Aug. 26, 1919.

Applicationi`1ed-March 21, 1919. Serial No. 284,023.

cation, in whichand its high heat conductivity.

Figure 1 is a horizontal cross section through the furnace, and A Fig. 2 is a vertical cross section.

The invention relates to electric furnaces and more particularly to electric resistance furnaces suitable for th'e heating, harde-ning and tempering of small metal parts and for other purposes requiring a close regulation of temperature in a neutral or reducing atmosphere.

The object of the invention is to provide a furnace of the character herein described in which a high temperature is obtainable withoutl rapid deterioration of the furnace. Vith this object in view, the present invention consists in the electric furnace hereinafter described and particularly pointed out in the claims.

Referring to the illustrated embodiment of the invention z--The furnace comprises a muffle 1 composed of silicon carbid open at one end and constituting the furnace chamber in which are placed the objects to be heated. Silicon carbid is particularly adapted for this purpose on account of its refractoriness, its resistance to oxidation Several forms of silicon carbid may be used but we prefer to use a mufl'le composed of a material commercially known as silfraX, the muiile being molded from silicon carbid grains or powder and heated in a silicon atmosphere in an electric furnace .so that the grains are welded together in a solid mass of silicon carbid without any other bonding material. The walls of the muflle are preferably about one-half inch in thickness. Silicon carbid is formed at a temperature of about 1800O C. and is decomposed at 2200o C. It is therefore far more refractory than any of the ordinary mules made of fire clay. It has a thermal conductivity `of about eight times that of fire clay and this p is of prime importance in a refractory such as a muie which is used for the transmission of heat.

A mass of resistance material 2, preferably composed of granular carbon, graphite or coke, surrounds the muffle on all sides eX- cept at one end where the muflle is open and is carried through the outside casing of the furnace. The-resistance material is in direct contact with the muiile.

Graphite electrodes 3, 3 are connected at their outer ends outside of the casing to suitable sources of electrical energy, and eX- v tend inward and make contactw'ith the resistance material 2 so that When power is put on the furnace, the current enters the furnace through one electrode, traverses the.

ing the inutile up to thedesired temperature depending on the power employed.

Protection slabs of silicon carbid indicated by reference numerals 4, 4, 5, 5, 10 and 11 entirely surround the resistance material. Their function is to form a closed, solid receptacle which keeps the resistance material in a fixed position. These slabs are` pre-ferably about one inch thick. We use for this purpose a form of silicon carbid refractory commonly known as carbofrax consisting of silicon carbid grains bound together by a binder of clay and fired in themanner of fire brick. The endA slabsl, 4fholding the electrodes, are recessed as shown and the side slabs 5, 5 are fittedA into the recesses. Iron bolts 6, 6 are provided to hold the two end slabs together, thus firmly binding the slabs together. These bolts are placedclose to the outside casing o-f the furnace so that they are not subjected to any considerable temperature. The bottom slab 10 is supported by means of. refractory blocks 12 resting on the bottom member of the outside casing of the furnace. The muHle 1 is supported on the slab 10 by means of the refractory blocks 13. The electrodes pass through the end slabs and are provided on their inner ends with flanges which set up this insulating material rfil y of the furnace. The outside case 8 is conthe interior of the muflie.

small furnace of this type having a muflle.

structed preferably of asbestos board or a .similar material in the form of a rectangular box. The muiile being o en at one end is provided witha fire clay block 9 closel fitting the inside of the muflle and forming a door which can be removed to gain access to In operating a measuring 6 inches by 6 inches by 18 inches in length, we 'have used a layer of resistance material around the mufile one inch in thick- ,ness To 'attain a temperature vof 15000 C.,

the power consumed was approximately nine kilowatts at'twenty volts.

The advantages of this furnace overother n types of mufile furnaces heated by oil gas or electricity for use for similar purposes are durability, close regulation of temperature and the high temperature range obtainable without rapid deterioration from excessive heating. The granular carbon resistor is an ideal material for resistance purposes in an electric furnace on account of its cheapness.

and its uniformity of resistance and uniform heating effec-t. In furnaces where a carbon resistance material comes injcontact with a fire clay muffle, or other refractory contaminating clay in considerable amount, the upper temperature limit is very definitely .fixed because the carbon will react with the constituents of the clay if the temperature is too high, and as a practical matter it 1s found that a range of'working temperature `inside ythe mufile up to 1500O C. is not possible With this construction, .because this means' a much higher temperature on the side of the mufile in contact with the carbon. Vhen carbon is brought in contact with a silicon carbid mufiie, all these difficulties disappear and not only is there no re-action between the two substances, but silicon carbid is Vcapable of transmitting the heat most efficiently and having a very low co-efiicient of expansion, it does notl crack and deteriorate with alternate heating and cooling such as is necessary in opening and closing the furnace. A carbon resistance material must of course be kept isolated from the air to prevent oxidation and this is effectively accomplished by the protection slabs 4, 4 which are closely fitted together and surrounded by the insulating material, thus making access from the outside air substantially lmpossible. We have found that the resistance material does not need to be renewed oftener than once every month or two and this -can be readily done by removing the top of the casing and then removing the top protection slab 11. This furnace has a neutral or reducing atmosphere. This is a great advantage 1n heating steel parts which do not undergo any surface oxidation.

While the preferred embodiment of the present invention has been specifically illustrated and described, it is to be understood that the present invention is not limited to the illustrated details of construction but may lbe otherwise embodied within the scope of the following claims.

We claim: 1. An electric resistance furnace comprising a silicon carbid muiie and current conducting resistance material around the mufile; substantially as described.

2. An electric resistance furnace comprising a silicon carbid mufile, current conducting resistance materials around the muiile, and a closed refractory receptacle surrounding the material for keeping it from contact with the air; substantially as described.

3. An electric resistance furnace comprising a silicon carbid 'mufile, a `current conducting Vresistance material around the mufile, and a closed silicon carbid-refractory receptacle surrounding the'resistance material for keeping it', from contact with the air; substantially as described.

4. An electric resistance furnace comprising a silicon carbid mule, a current conducting resistance material around the mufiie, a closed silicon carbid refractory receptacle surrounding the resistance material for keeping it from contact with the air, and a heat-insulatin casing surroundin the said receptacle; su stantially as descri ed.

In testimony whereof, We have hereunto set our hands.

JOHN FITZPATRICK. LEVI THURSTON STEPHENS. 

